The old alluvium formed in early Pleistocene in San Juan, Puerto Rico has undergone substantial post-depositional weathering in the tropical marine climate, resulting in a special combination of mineralogy and microstructure, The stiff intact material consists of interconnected silt-sized aggregates, which in turn comprise groups of clay platelets containing smectites. Finely divided Fe-oxides act as coatings over clay platelets and bridge connections between aggregates so that a stiff network of aggregates is formed in the intact material. Such microstructure with Fe-oxides cementation renders this soil unusual deformation characteristics. The consolidation behaviour differs significantly from that of common sedimentary soil : (1) the coefficient of consolidation decreases from 0.1 to 10-5cm2\ / s by four orders of magnitude as the soil is compressed to 20 MPa ; (2) the swelling strain increases significantly with the maximum past consolidation stress ; (3) the intact soil exhibits exceptionally high yield stresses (e. g., 0.8 MPa and 6 MPa for the upper and lower layers, respectively) ; and (4) the normal compression strain can be completely recovered upon unloading when samples are pre-compressed beyond the yield stress. Results of triaxial drained shear tests suggest that the intact soil possess isotropic Mohr-Coulomb strength parameters and a nearly constant cohesion over depth, despite variations in mineralogy and density with depth. Such abnormal deformation characteristics are the macroscopic exhibitions of microstructure alteration caused by stresses.